Currently, an optical recording medium, such as a CD (Compact Disc) or a DVD (Digital Versatile Disc), has been widely used as a medium for recording a large amount of data. In recent years, there have been a demand for recording a high-definition moving image and a demand for recording a larger amount of data along with the development of the personal computer. The optical recording medium, such as the CD or the DVD, cannot be sufficiently increased in density to allow such a large amount of data to be recorded thereon. Thus, the large amount of data cannot be recorded on a single disc. To record the large amount of data, therefore, it is necessary to record the data while replacing a plurality of discs. In recent years, attention has been drawn to a hologram recording medium which can record thereon a substantially larger amount of data than the existing CD and DVD.
As hologram information recording, there is a method of dividing beam light into two mutually coherent lights, performing data modulation on one of the lights in a spatial light modulator to form signal light, and combining, on a recording medium, the signal light with the other light which acts as reference light, to thereby record data as interference fringes. The recording medium is then irradiated with the same reference light as the reference light used in the recording so that the signal light can be detected as reproduction light. Thereby, the data can be reproduced. In the hologram information recording/reproducing, the signal light is not reproduced if the wavelength or the irradiation angle of the light used in the information reproduction is different from the wavelength or the irradiation angle of the light used in the information recording. With the use of the above characteristic, a recording has been performed in which the wavelength or the irradiation angle of the light used in the information recording/reproducing is changed to perform multiplex recording of data in the same area of the recording medium.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-216359